Memory device and method for operating the same including setting a recovery voltage

ABSTRACT

A memory device can include a plurality of memory cells including a first group of memory cells and a second group of memory cells programmed to a predefined logic state. The plurality of memory cells includes a memory controller configured to apply a reading voltage to at least one selected memory cell of the first group during a reading operation, apply the reading voltage to the memory cells of the second group, and responsive to the logic state of at least one memory cell of the second group being assessed to be different from the predefined logic state perform a refresh operation of the memory cells of the first group by applying a recovery voltage higher than the reading voltage to assess the logic state thereof and reprogramming the memory cells of the first group to the logic state assessed with the recovery voltage.

TECHNICAL FIELD

The present invention relates to the field of electronics, and morespecifically to an electronic memory device and to a method of operatingsaid memory device.

BACKGROUND ART

Electronic memory devices (hereinafter, briefly referred to as “memorydevices”) are widely used to store data in various electronic devicessuch as tablets, computers, wireless communication devices (e.g.,smartphones), cameras, digital displays, and the like.

Memory devices comprise a plurality of memory cells adapted to storedata in the form of programmable logic states. For example, binarymemory cells can be programmed into two different logic states, oftendenoted by a logic “1” (also referred to as “SET” state) or a logic “0”(also referred to as “RESET” state). In other systems, more than twologic states may be stored. To access the stored data, a module/unit ofthe electronic device may read, or sense, the stored logic state in thememory device. To store data, a module/unit of the electronic device maywrite, or program, the logic state in the memory device.

Memory devices may be of the non-volatile type or may be of the volatiletype. A non-volatile memory device comprises memory cells that arecapable of retaining the stored data by maintaining their programmedlogic state for extended periods of time even in the absence of anexternal power source. A volatile memory device comprises memory cellsthat may lose their stored data over time unless they are periodicallyrefreshed by an external power source.

Several kinds of non-volatile memory devices are known in the art, anon-exhaustive list thereof comprising read-only memory devices, flashmemory devices, ferroelectric Random Access Memory (RAM) devices,magnetic memory storage devices (such as for example hard disk drives),optical memory devices (such as for example CD-ROM disks, DVD-ROM disks,Blu-ray disks), Phase Change Memory devices (PCM).

PCM memory devices comprise memory cells each one including aphase-change material element that can be reversibly switched between anamorphous phase and a crystalline phase. The present disclosure relatesto improvements in such kind of memory device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a memory device in which a solutionaccording to embodiments of the present invention can be applied;

FIG. 2 illustrates in greater detail a portion of an exemplary array ofmemory cells of the memory device of FIG. 1;

FIG. 3 illustrates exemplary threshold voltage distributions of memorycells of the memory device of FIG. 1;

FIG. 4 illustrates in terms of functional blocks a recovery procedurefor the memory device of FIG. 1 according to embodiments of theinvention;

FIG. 5 illustrates the evolution of a test reading voltage duringiterations of a recovery voltage setting operation according toembodiments of the invention;

FIG. 6 illustrates an example of an electronic apparatus comprising thememory device of FIG. 1 according to embodiments of the invention.

DETAILED DESCRIPTION

A phase-change material element exhibits different electric resistivityvalues depending on its phase, which can be associated to correspondingdifferent logic states. The resistivity of the phase-change material inthe amorphous phase is higher than the resistivity of the material inthe crystalline phase. Different degrees of partial crystallization canalso be possible, having intermediate resistivity values between the oneof the (fully) amorphous phase and the one of the (fully) crystallinephase.

Ideally, all memory cells of a PCM memory device (hereinafter, brieflyreferred to as “PCM cell”) should feature a same (nominal) resistivity(and therefore a same threshold voltage, the latter being the voltage tobe applied to the memory cells for causing them to conduct a electriccurrent) for a same logic state. However, since different PCM cellsprogrammed to a same logic state practically exhibit differentresistivity values because of several factors (such as for examplevariations in the electrical characteristics of the phase-changematerial caused by the execution of a number of read-write operationsand/or by manufacturing tolerances), each logic state is actuallyassociated to a respective resistivity distribution (typically aGaussian-type distribution), and therefore to a respective thresholdvoltage distribution.

In order to assess the logic state of a PCM cell, a reading operation iscarried out directed to assess to which threshold voltage distributionthe threshold voltage of the PCM cell belongs. For example, a readingvoltage may be applied to the PCM cell and the logic state of the PCMcell is assessed based on (the presence or absence of) a currentresponsive to said reading voltage, the (presence or absence of the)current depending on the threshold voltage of the PCM cell. It should beunderstood that a cell thresholds (e.g., it becomes conductive) when avoltage difference is applied between its two terminals; such a voltagedifference may be obtained in different ways, for example biasing oneterminal, such as a word line terminal, to a negative voltage, such as aselection voltage, and the other terminal, such as a bit line terminal,to a positive voltage, such as a reading voltage. Other biasingconfigurations may produce the same effects (e.g., both the word lineand the bit line terminal biased to positive voltage, or the word lineterminal biased to a reference voltage, e.g. a ground voltage, and thebit line terminal biased to a positive voltage, for example). Forimproved clarity but without any limitation, in the followingdescription reference is made to the former case, in which the addressedword line is biased to a (negative) selection voltage (often notexplicitly mentioned), lower than a (positive) reading voltage used tobias the addressed bit line. Other access lines, e.g. unaddressed wordlines and unaddressed bit lines may be biased to intermediatedeselection voltages (e.g., a ground voltage or a positive voltage), asbetter explained below.

Making reference to a binary PCM memory device, in which two thresholdvoltage distributions are provided (for example a first thresholdvoltage distribution corresponding to the SET state and a secondthreshold voltage distribution corresponding to the RESET state, whereinthe threshold voltages of the first threshold voltage distribution arelower than the threshold voltages of the second threshold voltagedistribution), the value of the reading voltage is advantageouslyselected to be higher than the highest threshold voltage of the firstthreshold voltage distribution and lower than the lowest thresholdvoltage of the second threshold voltage distribution.

PCM memory devices are negatively affected by a drawback caused by thechange (in jargon, “drift”) experienced by the resistivity of a PCM cellas time passes after its last programming. Indeed, once a PCM cell hasbeen programmed to a logic state, corresponding to a resistivity value,the resistivity of the cell tends to increase with the passage of time,in a way that depends on several factors, such as the operatingtemperature of the PCM memory device (the higher the temperature, thefaster is the resistivity increase with time), and the resistivitycorresponding to the programmed logic state (PCM cells programmed tohigher resistivity values experience a faster resistivity time driftcompared to PCM cells programmed to lower resistivity values). Theresistivity time drift causes in turn a drift of the threshold voltagedistributions, which correspondingly move as time passes since the lastprogram operation.

If the drift of the threshold voltage distributions is particularly highbecause the PCM memory device has not been subjected to programoperations for a long time, the value of the reading voltage previouslyselected for carrying out reading operations could no longer be capableof assessing which threshold voltage distribution the threshold voltageof the PCM cell belongs to.

A solution known in the art provides for defining in advance (e.g.,during the design phase of the PCM memory device) a group of (e.g.,three) different reading voltages. Each reading voltage of the group isset in such a way to be used for a corresponding period of time whichwill occur after the last program operation and takes into account thedrift of the threshold voltage distributions expected during that periodof time.

Since according to the solutions known in the art a group comprising afinite number of reading voltages is provided, a situation will occur inwhich, after a certain time since the last program operation, thethreshold voltage distributions will be shifted to an extent such thatnone of the reading voltages of the group will be capable of assessingwhich threshold voltage distribution the threshold voltage of a memorycell belongs to.

In other words, with a finite number of reading voltages, the dataretention time (i.e., the time interval—after the last programoperation—for which a stored logic state is not lost because of drift)is limited.

The applicant has devised an alternative solution for a PCM memorydevice and for its operation.

An aspect of the present invention relates to a memory device. Accordingto embodiments of the present invention, the memory device comprises aplurality of memory cells and each memory cell is programmable to atleast two logic states.

According to embodiments of the present invention, each logic statecorresponds to a respective nominal electric resistance value of thememory cell and the plurality of memory cells comprises a first group ofmemory cells and a second group of memory cells.

According to embodiments of the present invention, the memory cells ofthe second group are programmed to a predefined logic state of said atleast two logic states.

According to embodiments of the present invention, the memory devicefurther comprises a memory controller coupled to the plurality of memorycells. Moreover, the memory controller is configured to apply a readingvoltage to at least one selected memory cell of the first group during areading operation to assess the logic state thereof according to acurrent responsive to said applied reading voltage.

The memory controller is also configured to apply the reading voltage tothe memory cells of the second group to assess the corresponding logicstate.

According to embodiments of the present invention, the memory controlleris further configured to, if the logic state of at least one memory cellof the second group is assessed to be different from said predefinedlogic state, perform a refresh operation of the memory cells of thefirst group.

According to embodiments of the present invention, the memory controlleris configured to perform a refresh operation of the memory cells of thefirst group by applying a recovery voltage higher than the readingvoltage to assess the logic state thereof and then reprogramming thememory cells of the first group to the logic state assessed with therecovery voltage.

According to embodiments of the present invention, said recovery voltagehas a predetermined value.

According to embodiments of the present invention, the memory controlleris further configured to carry out a recovery voltage setting operationfor setting said recovery voltage. Such a recovery voltage settingoperation comprises the following sequence of phases:

a) setting an initial test voltage;

b) applying a test voltage to the memory cells of the second group toassess the logic state thereof;

c) if the logic state of at least one memory cell of the second groupassessed with the test voltage is different from said predefined logicstate, increasing the value of the test voltage and repeating phase b)using said increased value of the test voltage;

d) if the logic state of all the memory cells of the second groupassessed with the test voltage is equal to said predefined logic state,setting said recovery voltage according to the test voltage used in thelast phase b) that has been carried out.

According to embodiments of the present invention, the initial testvoltage corresponds to said reading voltage and the memory controller isconfigured to set said recovery voltage to the test voltage used in thelast iteration of phase b).

According to embodiments of the present invention, after the firstiteration of phase b), the increased test voltage is applied to a subsetof the memory cells of the second group, namely to those memory cellsfor which the logic state has been assessed to be different from saidpredefined logic state; in other words the cells of the second groupthat have been assessed to have a logic state equal to the predefinedlogic state are masked during subsequent applying a test voltage to thememory cells of the second group to assess the logic state thereof (themeaning of which, therefore, includes both the case in which the testvoltage is applied to all the cells of the second group and the case inwhich the test voltage is applied only to a subset of the cells of thesecond group).

According to embodiments of the present invention, the memory controlleris further configured to apply the reading voltage to the memory cellsof the second group to assess the logic state thereof at each power-onof the memory device.

According to embodiments of the present invention, the memory device isa non-volatile memory device and each memory cell comprises a logicstate storage element including a phase change material.

Said phase change material is a chalcogenide material.

According to embodiments of the present invention, the memory controlleris further configured to:

-   -   if the logic state of at least one memory cell of the second        group is assessed to be different from said predefined logic        state, reprogram the memory cells of the second group to said        predefined logic state before performing said refresh operation.

The reading voltage is selected among a group of predefined readingvoltages based on the last time a memory cell of the first group hasbeen programmed. The cells of the memory device of the presentdisclosure are arranged in at least one array of memory cells with thecells of each array being arranged in a plurality of rows and aplurality of columns. This memory device further includes a plurality ofword lines and a plurality of bit lines, the memory cells of each rowbeing connected to a corresponding word line and the memory cells ofeach column being connected to a corresponding bit line, the memorycontroller being configured to select a memory cell connected to acorresponding word line and to a corresponding bit line for assessingthe logic state thereof by:

-   -   during a reading operation, biasing the corresponding bit line        to a voltage corresponding to the reading voltage, biasing the        corresponding word line to a word line selection voltage lower        than the reading voltage, and biasing the other word lines to a        first deselection voltage intermediate between the word line        selection voltage and the bit line reading voltage;    -   during the recovery voltage setting operation, biasing the        corresponding bit line to a voltage corresponding to the test        voltage, biasing the corresponding word line to the word line        selection voltage, and biasing the other word lines to a second        deselection voltage intermediate between the word line selection        voltage and the bit line reading voltage and higher than the        first deselection voltage;    -   during the refresh operation, biasing the corresponding bit line        to a voltage corresponding to the recovery voltage, biasing the        corresponding word line to the word line selection voltage, and        biasing the other word lines to the second deselection voltage.

Another aspect of the present invention relates to an electronicapparatus including at least: a processor module, a memory module aspreviously disclosed, a communication module and a possible a peripheralmodule.

FIG. 1 shows an exemplary memory device 100 according to embodiments ofthe present invention.

The memory device 100 is schematically illustrated in FIG. 1 in terms offunctional units/modules/blocks/components. According to embodiments ofthe invention, the memory device 100 is a non-volatile memory device.

According to exemplary embodiments of the invention, the memory devicemay be comprised in various electronic apparatuses and may be used tostore data, such as user and/or system data. For example, saidelectronic apparatuses may include tablets, computers, wirelesscommunication devices (e.g., smartphones), cameras, digital displays,and the like.

The memory device 100 comprises a plurality of memory cells 105 each oneadapted to be programmed to different logic states. According toembodiments of the invention, the memory cells 105 are arranged into anarray 106.

In the exemplary embodiments of the invention herein described indetail, each memory cell 105 is a binary memory cell that can beprogrammed to two different logic states, denoted as logic “1” (or SETstate) and logic “0” (or RESET state). In any case, the conceptsaccording to embodiments of the present invention can be also applied tothose cases in which each memory cell is a multi-level memory celladapted to be programmed to more than two different logic states.

The memory cells 105 are for example arranged in a plurality of rows anda plurality of columns. Each row of memory cells 105 is electricallycoupled to a word line 110, and each column of memory cells 105 iselectrically coupled to a bit line 115. Word lines 110 and bit lines 115may be substantially perpendicular to one another. It is pointed outthat references to word lines and bit lines, or their analogues, areinterchangeable without loss of understanding or operation. Word lines110 and bit lines 115 are conductive lines that may be made ofconductive materials such as metals (e.g., copper (Cu), aluminum (Al),gold (Au), tungsten (W), titanium (Ti)), metal alloys, carbon,conductively-doped semiconductors, or other conductive materials,alloys, compounds, or the like.

Generally speaking, a generic memory cell 105 of the array 106 islocated at the intersection of two corresponding conductive lines, andparticularly at the intersection of a corresponding word line 110 and acorresponding bit line 115. This intersection defines the address of thememory cell 105.

In order to select a set of memory cells 105 located at intersections ofconductive lines (such as one word line 110 and one or more bit lines115) for performing operations thereon (such as for performing a readingor a writing operation), said conductive lines are suitablyenergized/biased with corresponding biasing or selection voltages.

According to embodiments of the present invention, each memory cell 105comprises a logic state storage element comprising a material with avariable resistance. Materials with variable resistance may includevarious material systems, including, for example, metal oxides,chalcogenides, and the like. According to embodiments of the presentinvention, the logic state storage element is positioned between a firstelectrode and a second electrode of the memory cell 105. According toembodiments of the invention, one side of the first electrode iselectrically coupled to a word line 110 and the other side of the firstelectrode is electrically coupled to the logic state storage element ofthe memory cell 105. According to embodiments of the invention, one sideof the second electrode is electrically coupled to a bit line 115 andthe other side of the second electrode is electrically coupled to thelogic state storage element of the memory cell 105.

According to embodiments of the present invention, the memory device 100is a PCM memory device. According to embodiments of the presentinvention, the variable resistance material included in the logic statestorage element of each memory cell 105 comprises chalcogenide materialsor alloys that include at least one of the elements sulfur (S),tellurium (Te), or selenium (Se). Many chalcogenide alloys may bepossible—for example, a germaniumantimony-tellurium alloy (Ge—Sb—Te) isa chalcogenide material. Other chalcogenide alloys not expressly recitedhere may also be employed.

According to embodiments of the present invention, the logic statestorage element of a memory cell 105 is electrically coupled to the bitline 115 by a selector element. The word line 110 may be connected toand may control the selector element.

According to embodiments of the present invention, the selector elementis a selector transistor. The word line 110 may be connected to the gateof the selector transistor, Energizing the word line 110 results in anelectrical connection between the logic state storage element of amemory cell 105 and its corresponding bit line 115. The bit line 115 maythen be accessed to either read or write the memory cell 105.

According to an alternative embodiment of the invention, the selectorelement may comprise an electrically non-linear component (e.g., anon-Ohmic component) such as a metal-insulator-metal (MIM) junction, anOvonic threshold switch (OTS), or a metalsemiconductor-metal (MSM)switch, among other types of two-terminal selector elements such as adiode. According to embodiments of the invention, the selector elementmay comprise a chalcogenide alloy. For example, the selector element maycomprise an alloy of selenium (Se), arsenic (As), silicon (Si), andgermanium (Ge). According to embodiments of the present invention, thelogic state storage element and the selector element may be a soleelement. In other words, only one element acting both as selector and asstorage elements may be coupled between a first electrode and a secondelectrode of the memory cell 105; the selector and storage element maycomprise a chalcogenide material in some examples.

The access to the memory cells 105 may be controlled through a rowdecoder 120 and a column decoder 130. For example, a row decoder 120 mayreceive a row address from a memory controller 140 and energize acorresponding word line 110 according to the received row address.Similarly, a column decoder 130 may receive a column address from thememory controller 140 and accordingly energize a corresponding set ofbit lines 115.

Upon accessing, a memory cell 105 may be read, or sensed, by a senseunit 133 electrically coupled to the bit lines 115 to assess the storedlogic state of the memory cell 105. For example, a reading voltage maybe applied to a memory cell 105 (using the corresponding word line 110and bit line 115) and the logic state thereof is assessed based on aresulting electric current responsive to said reading voltage. Saidelectric current depends on a threshold voltage of the memory cell 105determined by the electrical resistance of the logic state storageelement. For example, according to embodiments of the invention a firstlogic state (e.g., SET state) may correspond to a finite amount ofcurrent, whereas a second logic state (e.g., RESET state) may correspondto no current or a negligibly small current. Alternatively, according toanother embodiment of the present invention, a first logic state maycorrespond to a current higher than a current threshold, whereas asecond logic state may correspond to a current lower than the currentthreshold.

In some cases, such as for example when the memory cells 105 aremulti-level memory cells adapted to be programmed to more than twodifferent logic states, two or more reading voltages may be applied. Forexample, a sequence of reading voltages may be applied until a currentis detected by sense unit 133.

According to embodiments of the present invention, the sense unit 133may include various transistors or amplifiers in order to detect andamplify a difference in the signals (e.g., currents) received fromselected bit lines 115, and assess the logic states. The assessed (bythe sense unit 133) logic state(s) of selected memory cell(s) 105 maythen be routed by the memory controller 140 to the output of the memorydevice 100 through an input/output unit 143.

According to the illustrated embodiment of the present invention, thesense unit 133 is distinct from and not directly connected to the columndecoder 130 and the row decoder 120. According to other (notillustrated) embodiments of the invention, the sense unit 133 may bepart of a column decoder 130 or row decoder 120, or the sense unit 133may be connected to or in electronic communication with column decoder130 or row decoder 120. The sense unit may include less amplifiers thanthe number of bit lines 115 and the amplifier(s) may be selectivelycoupled to one of several bit lines during an access operation, forexample.

Similarly, memory cells 105 may be set or programmed to chosen logicstates by energizing a selected word line 110 and one or more selectedbit lines 115 through the column decoder 130 and row decoder 120, forexample using data received through the input/output unit 143.

Generally, the memory controller 140 is configured to control theoperation (e.g., read, program, re-write, refresh) of the memory device100 through the row decoder 120, column decoder 130, and sense unit 133.According to various embodiment of the present invention, one or moreamong the row decoder 120, column decoder 130, and sense unit 133 may beco-located with the memory controller 140. According to embodiments ofthe present invention, the memory controller 140 is configured togenerate row and column address signals in order to energize the desiredword line 110 and bit line 115. According to embodiments of the presentinvention, the memory controller 140 may also generate and controlvarious voltages (e.g., reading voltages) or currents used during theoperation of memory device 100.

According to the illustrated embodiments of the invention, the array 106of memory cells 105 is a 3D array. However, it is pointed out that theconcepts of the present invention can be applied as well to a memorydevice 100 having the memory cells 105 arranged in 2D arrays only.

According to the illustrated embodiments of the invention, the memorycells 105 are arranged according to the 3D XPoint™ (also referred to ascross-point) architecture.

In the exemplary embodiment illustrated in the figures, the 3D array 106comprises two two-dimensional (2D) memory arrays formed adjacent oneanother forming two levels of memory cells 105 stacked to each other.

However, similar considerations apply in case the 3D array 106 comprisesmore than two 2D memory arrays stacked to each other.

The memory cells 105 of each level are for example arranged in aplurality of rows and a plurality of columns.

The 3D architecture allows to advantageously increase the number ofmemory cells 105 that may be placed or created on a single die orsubstrate as compared with 2D architectures. The 3D architecture maythus reduce production costs, or increase the performance of the memorydevice, or both. In the considered example, each one of the two levelsof memory cells 105 is aligned or positioned such that a pair of memorycells 105 may be aligned (exactly, overlapping, or approximately) withone another across each level, forming a corresponding memory cell stack145.

In the embodiment illustrated in FIG. 1, the two memory cells 105 of amemory cell stack 145 share a common conductive line such as a same bitline 115. In other words, for each memory cell stack 145, a same bitline 115 may be electrically coupled with a bottom electrode of theupper memory cell 105 in the memory cell stack 145 and a top electrodeof the lower memory cell 105 in the memory cell stack 145.

However, similar considerations apply to other embodiments in which eachmemory cell 105 in a memory cell stack 145 (e.g., the upper one, thelower one) is electrically coupled with a respective different bit line.In this case, the memory cells can be separated by means of a dedicatedan insulation layer.

FIG. 2 illustrates in detail an exemplary portion 200 of the array 106of the memory device 100 illustrated in FIG. 1. Particularly, theportion 200 illustrated in FIG. 2 comprises four adjacent memory cells105 belonging to the lower level of the 3D array 106, i.e., the memorycells 105 visible in FIG. 2 are the lower memory cells of fourrespective memory cell stacks 145 (the upper memory cells being notvisible in FIG. 2).

In the exemplary embodiment of the invention illustrated in FIG. 2, eachmemory cell 105 comprises three electrodes 205(1), 205(2), 205(3), alogic state storage element 210, and a selector element 220. Accordingto embodiments of the present invention, the electrode 205(1) has afirst end connected to a bit line 115 and a second end connected to theselector element 220. According to embodiments of the present invention,the selector element 220 is further coupled to the logic state storageelement 210 through the electrode 205(2). According to embodiments ofthe present invention, the electrode 205(3) has a first end connected tothe logic state storage element 210 and a second end connected to a wordline 110. Similar considerations apply in case the positions of thelogic state storage element 210 and of the selector element 220 areswitched.

According to embodiments, the logic state storage element 210 comprisesa variable resistance element including a phase-change material such asa chalcogenide alloy. In some embodiments, the selector element 220 mayinclude an electrically non-linear component, for example comprising achalcogenide alloy.

According to another embodiment which is not illustrated, a singlecomponent including a chalcogenide alloy could be used for replacing theselector element 220, the logic state storage element 210 and theelectrode 205(2).

As already mentioned above, and as known to those skilled in the art,the memory cell 105 can be programmed to different logic states byvarying the electrical resistance of the logic state storage element210, and thus causing a corresponding variation of the threshold voltageof the memory cell 105. For example, said varying the electricalresistance may be caused by forcing a current through the memory cell105 such to heat the logic state storage element 210 thereof.

For example, in order to program a memory cell 105 to a low-resistancestate (e.g., the SET state), current is forced to flow across the memorycell 105 for heating the logic state storage element 210 thereof untilthe logic state storage element 210 reaches a sufficiently hightemperature (but below its melting temperature). This causes in turn thecrystallization of the phase change material of the logic state storageelement 210.

In order to program a memory cell 105 to a high-resistance state (e.g.,the RESET state) current is forced to flow across the memory cell 105for heating the logic state storage element 210 thereof above itsmelting temperature, and then by abruptly removing the applied currentto let the logic state storage element 210 quickly cool down. In thisway, the phase change material of the logic state storage element 210takes an amorphous structure having a higher resistivity.

As already mentioned above, ideally, all the logic state storageelements 210 of the memory cells 105 of the memory device 100 shouldfeature, for a same logic state, a same (nominal) electrical resistanceand therefore a same threshold voltage. However, since logic statestorage elements 210 of different memory cells 105 of the memory device100 programmed to a same logic state practically exhibit differentelectric resistivity values, each logic state is associated to arespective threshold voltage distribution. For example, the thresholdvoltage distributions are Gaussian-type distributions.

Making reference to the diagram illustrated in FIG. 3, with reference302(0) it is depicted an exemplary threshold voltage distribution ofmemory cells 105 of the memory device 100 that have been programmed tothe SET state (hereinafter, “SET distribution”), while with reference304(0) it is depicted an exemplary threshold voltage distribution ofmemory cells 105 of the memory device 100 that have been programmed tothe RESET state (hereinafter, “RESET distribution”).

In the example illustrated in FIG. 3, the SET distribution 302(0) has alower edge corresponding to a voltage E1(0) and an upper edgecorresponding to a voltage E2(0), with E2(0)>E1(0). This means that,according to this example, the threshold voltages of the memory cells105 that have been programmed to the SET state have values comprised inthe interval [E1(0), E2(0)].

In the example illustrated in FIG. 3, the RESET distribution 304(0) hasa lower edge corresponding to a voltage E3(0) and an upper edgecorresponding to a voltage E4(0), with E4(0)>E3(0). This means that,according to this example, the threshold voltages of the memory cells105 that have been programmed to the RESET state have values comprisedin the interval [E3(0), E4(0)].

According to embodiments, in order to avoid that the SET distribution302(0) and the RESET distribution 304(0) overlaps, and therefore inorder to avoid the occurrence of wrong readings, the lower edge E3(0) ofthe RESET distribution 304(0) has to be sufficiently higher than theupper edge E2(0) of the SET distribution E2(0), so as to provide asufficiently large safe voltage interval between the two distributions.

The exemplary SET distribution 302(0) and RESET distribution 304(0) ofFIG. 3 illustrate an example of how the threshold voltages of the memorycells 105 are distributed across the memory cells 105 of the memorydevice 100 at a time t(0) corresponding for example to a last memorycell 105 program operation.

As already mentioned in the foregoing, during a reading operation, areading voltage may be applied to a target memory cell 105 to beaccessed and the logic state of the latter may be assessed based on aresulting current flowing across the target memory cell 105 responsiveto said reading voltage. Said current depends on the threshold voltageof the memory cell 105. For example, making reference to a readingoperation occurring not long after the last memory cell 105 programoperation (e.g., at a time not long after time t(0)), a reading voltageVR(0) may be used that is higher than the upper edge E2(0) of the SETdistribution E2(0) and at the same time is lower than the lower edgeE3(0) of the RESET distribution 304(0).

Making reference to the illustrated example, the following takes place.

If the logic state of the target memory cell 105 is the SET state, theapplied reading voltage VR(0) is higher than the threshold voltage ofthe target memory cell 105, which is comprised in the interval [E1(0),E2(0)]. In this situation, a finite current flows across the targetmemory cell 105.

If the logic state of the target memory cell 105 is the RESET state, theapplied reading voltage VR(0) is lower than the threshold voltage of thetarget memory cell 105, which is comprised in the interval [E3(0),E4(0)]. In this situation, no current flows across the target memorycell 105, or only a negligibly small current, substantially lower thanthe current corresponding to the case of the SET state.

As mentioned above, the resistivity of the phase-change material (e.g.,chalcogenide alloy) comprised in the logic state storage elements 210 ofthe memory cells 160 is subjected to an increase (drift) as time passessince the last program operation. The drift in resistivity causes inturn a drift of the threshold voltage distributions, whichcorrespondingly shift (toward higher voltages) as time passes since thelast program operation.

An example of how the SET distribution and the RESET distribution movebecause of the drift is illustrated in FIG. 3, wherein:

-   -   the SET distribution at a time t(1)>t(0) since a last memory        cell 105 program operation is identified with reference 302(1);    -   the SET distribution at a time t(2)>t(1) since a last memory        cell 105 program operation is identified with reference 302(2);    -   the RESET distribution at time t(1) is identified with reference        304(1);    -   the RESET distribution at time t(2) is identified with reference        304(2).

Making reference to the not limitative illustrated example, as timepasses since the last program operation, the upper edge of the SETdistribution and the lower edge of the RESET distribution increase.Indeed, the upper edge E2(1) of the SET distribution 302(1) at time t(1)is higher than the upper edge E2(0) of the SET distribution 302(0) attime t(0). The upper edge E2(2) of the SET distribution 302(2) at timet(2) is higher than the upper edge E2(1) of the SET distribution 302(1)at time t(1). The lower edge E3(1) of the RESET distribution 304(1) attime t(1) is higher than the lower edge E3(0) of the RESET distribution304(0) at time t(0). The lower edge E3(2) of the RESET distribution304(2) at time t(2) is higher than the lower edge E3(1) of the RESETdistribution 304(1) at time t(1).

As can be seen from the example illustrated in FIG. 3, if the timepassed since the last program operation is sufficiently large, the valueof the reading voltage VR(0)—set to be higher than the upper edge E2(0)of the SET distribution E2(0) and at the same time lower than the loweredge E3(0) of the RESET distribution 304(0)—could eventually be nolonger capable of fully discriminating between memory cells 105programmed to the SET state and memory cells 105 programmed to the RESETstate. For example, if the reading voltage VR(0) was used at time t(2),i.e., after that the time passed since the last program operation ist(2)−t(0), a fraction of memory cells 105 whose threshold voltagesbelong to the SET distribution 302(2), and particularly those cellswhose threshold voltages belong to the higher portion of the SETdistribution 302(2), would result to have threshold voltages higher thanthe reading voltage VR(0). In other words, a fraction of memory cells105 in the SET logic state would be incorrectly assessed to be in theRESET logic state.

For this reason, according to embodiments, a group of different readingvoltages VR(i) is defined in advance. According to embodiments, eachreading voltage VR(i) of the group is directed to be used only for acorresponding time interval T(i)=[t(i), t(i+1)) occurring after the lastprogram operation. According to embodiments, the value of each readingvoltage VR(i) is calculated in advance in such a way to be higher thanthe expected upper edge E2(i) of the SET distribution 302(i) and at thesame time lower than the expected lower edge E3(i) of the RESETdistribution 304(i) at a time t(i) since the last program operation.According to embodiments, the number of reading voltages VR(i), and/orthe values of the reading voltages VR(i), and/or the time intervals T(i)are set in advance during the design phase of the memory device 100.

Making reference to the exemplary case illustrated in FIG. 3, threereading voltages are considered:

-   -   a first reading voltage VR(0) is defined to be used for carrying        out reading operations during a first time interval T(0)        starting from the end of last program operation (time t(0));    -   a second reading voltage VR(1) (higher than the first reading        voltage VR(0)) is defined to be used for carrying out reading        operations during a second time interval T(1) starting from the        end of the first interval T(0);    -   a third reading voltage VR(2) (higher than the second reading        voltage VR(1)) is defined to be used for carrying out reading        operations during a third time interval T(2) starting from the        end of the second interval T(I).

According to this example, the reading voltage VR(1) is higher than theupper edge E2(1) of the SET distribution 302(1) and at the same timelower than the lower edge E3(1) of the RESET distribution 304(1), andthe reading voltage VR(2) is higher than the upper edge E2(2) of the SETdistribution 302(2) and at the same time lower than the lower edge E3(2)of the RESET distribution 304(2).

Given that a finite number of reading voltages VR(i) is provided (in theconsidered example, three), a situation will occur in which, after acertain amount of time since the last program operation, the SETdistribution will be shifted upward to such an extent to cross the last(and highest) reading voltage VR(i) of the group (in the consideredexample, the reading voltage VR(2)). In this situation, the value of thelast (and highest) reading voltage VR(i) of the group could no longer becapable of fully discriminating memory cells 105 programmed to the SETstate from memory cells 105 programmed to the RESET state. An example ofthis situation is illustrated in FIG. 3, wherein, at a time t(3)subsequent to time t(2), the corresponding SET distribution 302(3) isshifted upwards crossing the reading voltage VR(2).

In other words, with a finite number of reading voltages VR(i), the dataretention time (i.e., the time interval—after the last programoperation—for which a stored logic state is not lost because of drift)is limited.

Current minimum operative requirements for non-volatile memory devicesprovide for a data retention time of 7 years at a temperature of 40° C.for standard applications, and a data retention time of 5 years at atemperature of 55° C. for mobile applications.

While current PCM technology allows to fulfil the minimum operativerequirements for standard applications, the same technology cannotguarantee the desired data retention time at a temperature of 55° C. orhigher.

According to embodiments of the present invention which will bedescribed hereinbelow, a system and a method is provided for increasingthe data retention time of the memory device 100.

Returning to FIG. 1, according to embodiments of the invention, thememory device 100 further comprises a group of cells (hereinafterreferred to as “sentinel cells” and identified with reference 180) thatare programmed to a predefined logic state.

According to embodiments of the present invention, the predefined logicstate is the same for all the sentinel cells 180. According toembodiments of the present invention, the predefined logic state is thelogic state corresponding to the threshold voltage distributioncomprising the lowest threshold voltages. According to embodiments ofthe present invention, the predefined logic state is the SET logicstate.

According to embodiments of the present invention, the sentinel cells180 are selected during a design phase of the memory device 100.According to embodiments of the invention, the sentinel cells 180 aresparsely (e.g., randomly) located in the array 106 together with thememory cells 105. According to another embodiment of the presentinvention, the sentinel cells 180 are located in specific parts of thearray 106, such as for example at borders and/or at corners and/or inthe center of the array 160. According to embodiments of the invention,if the array 106 is a 3D array comprising a number of two-dimensional(2D) memory arrays forming a number of levels of memory cells 105stacked to each other, the sentinel cells 180 may be located in everylevel of memory cells 105, or they may be located in only a subset oflevels of memory cells 105 (e.g., only one).

According to embodiments of the present invention not illustrated in thefigures, the sentinel cells 180 are arranged in a dedicated arraydifferent from the array 106.

According to another embodiment of the present invention, instead ofbeing selected during a design phase of the memory device 100, thesentinel cells 180 are dynamically selected, for example randomly,during a first power on of the memory device 100. In this case, theaddresses of the sentinel cells 180 are stored in a correspondingregister or in an equivalent unit/module for allowing a selectionthereof.

According to embodiments of the present invention, the number ofsentinel cells 180 compared to the number of memory cells 105 depends onthe material used for the logic state storage elements 210 of the memorycells.

According to embodiments of the invention, the sentinel memory cells 180are structurally identical to the memory cells 105, the only differenceconsisting in that the sentinel cells 180 are programmed to a predefinedlogic state. According to embodiments of the present invention, thesentinel memory cells 180 are programmed in such a way that theirthreshold voltages are located in the upper portion of the thresholdvoltage distribution (i.e., the one with highest threshold voltages)corresponding to the predefined logic state.

According to another embodiment of the present invention, the sentinelcells 180 are manufactured in such a way to exhibit a slighter highernominal threshold voltage compared to the other memory cells 105.

Therefore, according to embodiments of the invention, the array 106comprises two groups of memory cells, i.e., a first group of “standard”memory cells 105 directed to store data, e.g., user data, and a secondgroup of sentinel cells 180 which are programmed to a predefined logicstate.

According to embodiments of the invention, the sentinel cells 180 areexploited during a recovery procedure directed to counteract the effectof the threshold voltage distribution drift.

According to some embodiments of the present invention illustrated inthe figures, the recovery procedure is carried out by the memorycontroller 140. On this regard, the memory controller 140 and/or atleast some portions thereof may be implemented in hardware, softwareexecuted by a processor module/unit, firmware, or any combinationthereof. For example, the processor module/unit may be general-purposeprocessor, a digital signal processor (DSP), an application-specificintegrated circuit (ASIC), an field-programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described in the present disclosure. According toother embodiments of the present invention not illustrated in thefigures, the recovery procedure is instead carried out at leastpartially by a dedicated module (implemented in hardware, softwareexecuted by a processor module/unit, firmware, or any combinationthereof) different from the memory controller 140.

The recovery procedure according to embodiments of the invention isshown in terms of functional blocks in the flow chart illustrated inFIG. 4 and it is globally identified with reference 400.

According to embodiments of the present invention, the recoveryprocedure 400 is started at each power-on of the memory device 100(block 405). According to other embodiments of the invention notillustrated, the recovery procedure may be also triggered at differenttimes, such as for example periodically and/or when the memory device100 is in a stand-by state.

According to embodiments of the present invention, the memory controller140 carries out a reading operation directed to assess the logic stateof the sentinel cells 180 (block 410).

For this purpose, according to embodiments of the present invention, allor some of the sentinel cells 180 are selected, and a correspondingvoltage is applied thereto.

According to embodiments of the invention, the voltage used to read thesentinel cells 180 is the highest reading voltage among the group ofreading voltages VR(i).

Making reference to the example at issue, in which the group of readingvoltages includes the three reading voltages VR(0), VR(1), VR(2), thesentinel cells 180 are read by exploiting the reading voltage VR(2).

According to another embodiment of the invention, the voltage used toread the sentinel cells 180 is a function of the highest reading voltageamong the group of reading voltages VR(i).

Making reference to the example at issue, the sentinel cells 180 may beread by exploiting a voltage that is a function of the reading voltageVR(2), such as for example a fraction of the reading voltage VR(2)(e.g., the 90% or 80% of the reading voltage VR(2)). In other examples,the sentinel cells may be read using a voltage that is smaller than thereading voltage VR(2) by a predefined quantity (e.g., the readingvoltage may be VR(2)−0.5V or VR(2)−2.0V). According to embodiments ofthe invention, once the logic state of the sentinel cells 180 has beenassessed, the memory controller 140 checks whether the read logic statesof all the sentinel cells 180 correspond to the predefined logic state(e.g., the SET state) or not (block 415).

Since the sentinel cells 180 are equal or at least similar to the memorycells 105, and are subjected to the same or to very similarenvironmental/electric/operative stimuli, according to embodiments ofthe present invention, the sentinel cells 180 are used as samples totest the actual condition (from the drift point of view) of the rest ofthe memory cells 105 of the array 106.

According to embodiments of the present invention, if all the readsentinel cells 180 are assessed to store the predefined logic state(e.g., the SET state), it means that the time passed since a lastprogram operation has not been sufficient to allow that the thresholdvoltage distributions drift to an extent such to compromise the resultof reading operations directed to the memory cells 105. In this case(exit branch N of block 415), no refresh operation is required, and thememory cells 105 can be subjected to standard (e.g., read and/orprogram) operations depending on the particular application of thememory device 100 (block 420).

According to embodiments of the present invention, if the logic state ofat least one sentinel cell 180 is assessed to be different from thepredefined logic state (e.g., if the assessed logic state of at leastone sentinel cell 180 is assessed to be different from the SET state,such as for example is assessed to be the RESET state), it means that inthe actual conditions, at least some of the memory cells 105 could haveexperienced a drift sufficiently large to compromise the result ofpotential reading operations directed to said memory cells 105. In thiscase (exit branch Y of block 415), a refresh operation is carried out,to reset the effect of the drift and push back the threshold voltagedistributions to “freshly-programmed” positions by recovering the logicstates of the memory cells 105 and then rewriting/reprogramming therecovered logic states to the same memory cells 105.

According to embodiments of the present invention, once the memorycontroller 140 has assessed that a refresh operation have to be carriedout, it sets a recovery voltage VREC (block 425) to be used forrecovering the logic states of the memory cells 105 of the array 106 ina safe way even if the threshold voltages of some of said memory cells105 have been affected by a serious drift.

According to embodiments of the present invention, the recovery voltageVREC is higher than the highest reading voltage among the group ofreading voltages VR(i) (e.g., higher than VR(2)).

According to embodiments of the present invention, the recovery voltageVREC is set to a fixed predetermined value, which is defined during adesign and/or test phase of the memory device 100. According toembodiments of the invention, the recovery voltage is selected to besufficiently higher than the highest reading voltage among the group ofreading voltages VR(i) to allow to correctly read the logic state ofthose memory cells 105 whose threshold voltages exceed the highestreading voltage among the group of reading voltages VR(i) because ofdrift.

According to another embodiment of the present invention, the recoveryvoltage VREC is dynamically set by taking into account the actualcondition of the memory cells 105 (from the drift point of view).According to embodiments of the present invention, the recovery voltageVREC is set through a recovery voltage setting operation to a value thatis sufficiently high to exceed the higher threshold voltages belongingto the lowest threshold voltage distribution (e.g., the SETdistribution), and at the same time that is not excessively high tocause other disturbance effects.

According to embodiments of the present invention illustrated in FIG. 5,the recovery voltage setting operation provides for an iteration ofreading operations using a test reading voltage TV that is incrementedat each iteration. According to embodiments of the present invention,the value of the test reading voltage TV of the first iteration is setto the highest reading voltage among the group of reading voltages VR(i)(e.g., VR(2)). According to another embodiment of the present invention,the value of the test reading voltage TV of the first iteration isinstead set to a value higher than the highest reading voltage among thegroup of reading voltages VR(i) (e.g., higher than VR(2)).

According to embodiments of the invention, at each iteration, the memorycontroller 140 applies the test reading voltage TV to the sentinel cells180 in order to assess the logic state thereof.

According to embodiments of the present invention, if the logic state ofat least one sentinel memory cell 180 assessed with the current testreading voltage TV is different from the predefined logic state (e.g.,different from the SET state), the test reading voltage TV is increasedby a fixed or variable amount, and a next iteration is carried out, byapplying the increased test reading voltage TV to the sentinel cells 180in order to assess the logic state thereof.

According to embodiments of the present invention, if the logic state ofall the sentinel memory cells 180 assessed with the current test readingvoltage TV is equal to the predefined logic state (e.g., equal to theSET state) the iteration of reading operations using the test readingvoltage TV is interrupted. As can be seen in the example illustrated inFIG. 5, the test reading voltage TV of the last iteration is thereforesufficiently high to exceed also the upper edge of the SET distributionformed by the threshold voltages of the sentinel cells 180 (identifiedin FIG. 5 with reference 510). In this situation, according toembodiments of the present invention, the memory controller 140 sets therecovery voltage VREC according to the test reading voltage TV of thelast iteration.

According to embodiments of the invention, the memory controller 140sets the recovery voltage VREC to the value of the test reading voltageTV of the last iteration.

According to another embodiment of the invention, the memory controller140 sets the recovery voltage VREC to a value that is function of, e.g.,it is based on, the value of the test reading voltage TV of the lastiteration, such as for example 5% or 10% more than the value of the testreading voltage TV of the last iteration. In another example, the memorycontroller 140 sets the recovery voltage VREC to a value that isfunction of the value of the test reading voltage TV of the lastiteration, such as for example 50 mV or 200 mV more than the value ofthe test reading voltage TV of the last iteration.

According to embodiments of the present invention, once the recoveryvoltage VREC has been set, the memory controller 140 rewrites/reprogramsthe sentinel cells 180 to the predefined logic state (e.g., the SETstate) (block 430). In this way, the effect of the drift on the sentinelcells 180 is reset, and the threshold voltages thereof are pushed backto approximately the values they had just after their last programoperation.

At this point, according to embodiments of the invention, the memorycells 105 of the array 106 are subjected to a refresh operationexploiting the recovery voltage VREC (blocks 435, 440).

According to embodiments of the invention, the refresh operationprovides for recovering the logic states of the memory cells 105 bycarrying out a read operation in which the logic states of the memorycells are assessed using the recovery voltage VREC as reading voltage(block 435). Since the sentinel cells 180 reflects the actual condition(from the drift point of view) of the rest of the memory cells 105 ofthe array 106, and since the recovery voltage VREC has been set in sucha way to be higher to exceed the upper edge of the SET distributioncorresponding to the threshold voltages of the sentinel cells 180, thereading operations carried out using the recovery voltage VREC have avery high probability of giving a correct result despite the presence ofheavy drifts.

According to embodiments of the invention, the recovered logic states ofthe memory cells 105 may be temporally stored in a buffer register, orin a different memory device, such as a flash memory device (notillustrated in the figures), or also in another portion of the memoryarray 106.

According to embodiments of the invention, the memory cells 105 are thenrewritten/reprogrammed to their corresponding recovered logic states(block 440). In this way, the effect of the drift on the memory cells105 is reset, and the threshold voltages thereof are pushed back toapproximately the values they had just after their last programoperation.

Once the refresh operation is carried out, the memory cells 105 can besubjected to standard (e.g., read and/or program) operations dependingon the particular application of the memory device 100 (go to block420).

According to the embodiments of the present invention described in thepresent disclosure, the memory cells 105, as well as the sentinel cells180 are selected by the memory controller 140 through the row decoder120 and the column decoder 130 to assess their logic states in differentsituations for different purposes.

According to embodiments of the present invention, the selection of amemory cell 105 during a standard reading operation may provide forbiasing the corresponding bit line 115 to a voltage corresponding to oneof the reading voltage among the group of reading voltages VR(i) (orVREC during a recovering phase after drift), and biasing thecorresponding word line 110 to a word line selection voltage VS lowerthan the reading voltage VR(i). According to embodiments of the presentinvention, the bit lines 115 corresponding to the unselected memorycells 105 may be biased to a first deselection voltage VD(1)intermediate between the word line selection voltage and the bit linereading voltage, such as for example the ground voltage. According toembodiments of the present invention, also the word lines 110corresponding to the unselected memory cells 105 may be biased to thefirst deselection voltage VD(1), such as for example to the groundvoltage.

Since the voltages used for accessing the memory cells 105 and/or thesentinel cells 180 during the refresh operations and the recoveryvoltage setting operations are higher than the reading voltages VR(i)used during standard reading operations, unwanted leakage losses couldbe experienced for memory cells that are not selected but thatcorrespond to the same word line and/or the same bit line of theselected memory cell(s). According to some embodiments of the presentinvention, in order to reduce said leakage losses, the unselected wordlines are advantageously biased to a second deselection voltage VD(2)higher than the first deselection voltage VD(1) during the recoveryvoltage setting operations and/or the refresh operations. According toembodiments of the invention, the selection of a memory cell 105 duringa recovery voltage setting operation may provide for biasing thecorresponding bit line 115 to a voltage corresponding to the testvoltage TV and biasing the corresponding word line 110 to a word lineselection voltage VS lower than the test voltage TV. According toembodiments of the present invention, the bit lines 115 corresponding tothe unselected memory cells 105 may be biased to a first deselectionvoltage VD(1) intermediate between the word line selection voltage andthe bit line reading voltage, such as for example the ground voltage.According to embodiments of the present invention, the word lines 110corresponding to the unselected memory cells 105 may be biased to asecond deselection voltage VD(2) higher than the first deselectionvoltage VD(1).

According to embodiments of the invention, the selection of a memorycell 105 during a refresh voltage setting operation may provide forbiasing the corresponding bit line 115 to a voltage corresponding to therecovery voltage VREC and biasing the corresponding word line 110 to aword line selection voltage VS lower than the recovery voltage VREC.According to embodiments of the present invention, the bit lines 115corresponding to the unselected memory cells 105 may be biased to afirst deselection voltage VD(1) intermediate between the word lineselection voltage and the bit line reading voltage, such as for examplethe ground voltage. According to embodiments of the present invention,the word lines 110 corresponding to the unselected memory cells 105 maybe biased to a second deselection voltage VD(2) higher than the firstdeselection voltage VD(1).

FIG. 6 illustrates in terms of functionalunits/modules/blocks/components an example of an electronic apparatus600 comprising the memory device 100 according to embodiments of theinvention.

The electronic apparatus 600 may be a tablet, a computer, a wirelesscommunication device, such as a smartphone, a camera, a digital display,and the like.

In the exemplary embodiment illustrated in FIG. 6, the electronicapparatus 600 may comprises one or more among processor module 610, amemory module 620, a communication module 630, and one or moreperipheral modules 640.

The processor module 610 may be a microprocessor, a digital signalprocessor, a microcontroller, or other units capable of executingsoftware and or firmware.

The memory module 620 may comprise one or more memory devices adapted tostore data, such as system data for the operation of the electronicapparatus and/or user data. For example, said one or more memory devicescomprise the memory device 100 according to embodiments of the inventiondescribed above. Those components are interconnected by a bus 650 orsimilar interconnection means.

The communication module 630 is configured to enable communicationsbetween the electronic apparatus 600 and other electronic apparatuses.For example, the communication module can comprise a Bluetooth®communication circuit, a Wi-Fi® communication circuit, and/or a radiocircuit configured to allow the electronic apparatus 600 to communicatewith other electronic devices over a mobile network connection.

The one or more peripheral modules 640 may comprise input/outputdevices, interface devices, and/or other peripheral devices, such as forexample a microphone, a keyboard, switches, a display, one or morespeakers, and so on.

The previous description presents and discusses in detail severalembodiments of the present invention; nevertheless, several changes tothe described embodiments, as well as different invention embodimentsare possible, without departing from the scope defined by the appendedclaims.

The invention claimed is:
 1. A memory device, comprising: a plurality ofmemory cells arranged in at least one array of memory cells, each of theplurality of memory cells programmable to at least two logic states,each logic state corresponding to a respective nominal electricresistance value of the memory cell, wherein the plurality of memorycells comprises: a first group of memory cells; and a second group ofmemory cells programmed to a predefined logic state of the at least twologic states; a memory controller coupled to the plurality of memorycells and configured to: apply a reading voltage to at least oneselected memory cell of the first group during a reading operation toassess the logic state thereof, wherein the logic state assessmentcomprises: biasing a corresponding bit line to a voltage correspondingto the reading voltage, biasing a corresponding word line to a word lineselection voltage lower than the reading voltage, and biasing other wordlines to a first deselection voltage intermediate between the word lineselection voltage and the bit line reading voltage; apply the readingvoltage to the memory cells of the second group to assess the logicstate thereof; and responsive to the logic state of at least one memorycell of the second group being assessed to be different from thepredefined logic state, perform a refresh operation of the memory cellsof the first group by: applying a recovery voltage higher than thereading voltage to assess the logic state thereof; and reprogramming thememory cells of the first group to the logic state assessed with therecovery voltage; carrying out, using the memory controller, a recoveryvoltage setting operation to set the recovery voltage, the recoveryvoltage setting operation comprising: setting an initial test voltage;applying a test voltage to the memory cells of the second group toassess the logic state thereof; responsive to the logic state of atleast one memory cell of the second group assessed with the test voltagebeing different from the predefined logic state: increasing the value ofthe test voltage; and repeating application of the test voltage to thememory cells of the second group to assess the logic state thereof usingthe increased value of the test voltage; and responsive to the logicstate of all the memory cells of the second group assessed with the testvoltage being equal to the predefined logic state, the memory controllersetting the recovery voltage according to the test voltage used in thelast application of the test voltage to the memory cells of the secondgroup to assess the logic state thereof that has been carried out. 2.The memory device of claim 1, wherein the recovery voltage has apredetermined value.
 3. The memory device of claim 1, wherein the memorycontroller is further configured to carry out a recovery voltage settingoperation to set the recovery voltage, the recovery voltage settingoperation comprising: setting an initial test voltage; applying a testvoltage to the memory cells of the second group to assess the logicstate thereof; responsive to the logic state of at least one memory cellof the second group assessed with the test voltage being different fromthe predefined logic state: increasing the value of the test voltage;and repeating application of the test voltage to the memory cells of thesecond group to assess the logic state thereof using the increased valueof the test voltage; and responsive to the logic state of all the memorycells of the second group assessed with the test voltage being equal tothe predefined logic state, setting the recovery voltage according tothe test voltage used in the last application of the test voltage to thememory cells of the second group to assess the logic state thereof thathas been carried out.
 4. The memory device of claim 3, wherein theinitial test voltage corresponds to the reading voltage.
 5. The memorydevice of claim 1, wherein the memory controller is configured to applythe reading voltage to the memory cells of the second group to assessthe logic state thereof at each power-on of the memory device.
 6. Thememory device of claim 1, wherein each memory cell of the plurality ofmemory cells comprises a logic state storage element including achalcogenide material.
 7. The memory device of claim 1, wherein,responsive to the logic state of at least one memory cell of the secondgroup assessed as different from the predefined logic state, the memorycontroller is configured to reprogram the memory cells of the secondgroup to the predefined logic state before performing the refreshoperation.
 8. The memory device of claim 1, wherein the reading voltageis selected among a group of predefined reading voltages based on a lasttime a memory cell of the first group was programmed.
 9. A method foroperating a memory device, comprising: applying a reading voltage to afirst group and a second group of memory cells of a memory component;and responsive to a logic state of at least one memory cell of thesecond group assessed as different from a predefined logic state,performing a refresh operation of the memory cells of the first group byapplying a recovery voltage higher than the reading voltage to assessthe logic state thereof; reprogramming the memory cells of the firstgroup to the logic state assessed with the recovery voltage; carryingout a recovery voltage setting operation to set the recovery voltage,the recovery voltage setting operation comprising: setting an initialtest voltage such that it corresponds to the reading voltage; applying atest voltage to the memory cells of the second group to assess the logicstate thereof; responsive to the logic state of at least one memory cellof the second group assessed with the test voltage being different fromthe predefined logic state: increasing the value of the test voltage;and repeating application of the test voltage to the memory cells of thesecond group to assess the logic state thereof using the increased valueof the test voltage; responsive to the logic state of all the memorycells of the second group assessed with the test voltage being equal tothe predefined logic state, setting the recovery voltage according tothe test voltage used in the last application of the test voltage to thememory cells of the second group to assess the logic state thereof thathas been carried out.
 10. The method of claim 9, wherein each memorycell comprises a logic state storage element including a chalcogenidematerial.
 11. The method of claim 9, wherein, responsive to the logicstate of at least one memory cell of the second group assessed asdifferent from the predefined logic state, the memory controller isconfigured to reprogram the memory cells of the second group to thepredefined logic state before performing the refresh operation.
 12. Themethod of claim 9, further comprising selecting the reading voltage isselected among a group of predefined reading voltages based on a lasttime a memory cell of the first group was programmed.
 13. A method foroperating a memory device, comprising: programming memory cells of asecond group of a plurality of memory cells of the memory device to apredefined logic state of at least two logic states, each logic statecorresponding to a respective nominal electric resistance value of theeach of the memory cells of the second group; during a readingoperation, applying a reading voltage to at least one selected memorycell of a first group of memory cells of the plurality of memory cellsto assess a logic state thereof; applying the reading voltage to thememory cells of the second group to assess the logic state thereof;responsive to the logic state of at least one memory cell of the secondgroup assessed as different from the predefined logic state: performinga refresh operation of the memory cells of the first group by applying arecovery voltage higher than the reading voltage to assess the logicstate thereof; and reprogramming the memory cells of the first group tothe logic state assessed with the recovery voltage; carrying out arecovery voltage setting operation to set the recovery voltage, therecovery voltage setting operation comprising: setting an initial testvoltage; applying a test voltage to the memory cells of the second groupto assess the logic state thereof; responsive to the logic state of atleast one memory cell of the second group assessed with the test voltagebeing different from the predefined logic state: increasing the value ofthe test voltage; and repeating application of the test voltage to thememory cells of the second group to assess the logic state thereof usingthe increased value of the test voltage; and responsive to the logicstate of all the memory cells of the second group assessed with the testvoltage being equal to the predefined logic state, setting the recoveryvoltage according to the test voltage used in the last application ofthe test voltage to the memory cells of the second group to assess thelogic state thereof that has been carried out.
 14. The method of claim13, wherein applying the recovery voltage comprises applying a recoveryvoltage having a predetermined value.
 15. The method of claim 13,further comprising setting the recovery voltage to a voltage based onthe test voltage used in a last iteration of application of the testvoltage to the memory cells of the second group.
 16. The method of claim13, setting the initial test voltage such that it corresponds to thereading voltage.
 17. The method of claim 13, further comprising applyingthe reading voltage to the memory cells of the second group to assessthe logic state thereof at each power-on of the memory device.